In the context of petroleum well drilling and completion it is conventional to provide a wellhead equipment assembly at the well mouth for controlling ingress to and egress from the well in the presence of substantial and changing pressures, elevated temperatures. It is conventional to run strings of casing and tubing into such a well for various purposes and to hang such strings from their upper end regions from within the wellhead equipment assembly. Sometimes, for that purpose the string is provided near its upper end with a hanger connected thereto, which hanger seats in the wellhead equipment assembly. In other instances the hanging is accomplished by depositing into the annulus between a seat in the bore of the wellhead equipment assembly and the string of pipe an annular wedge and then axially compressing that annular wedge (usually called pipe slips) to radially expand it securely into frictionally gripping relation with the string of pipe.
Generally, after setting force is applied to pipe slips, the means or device used to apply the setting force is withdrawn, since the slips once set tend to stay set due to the weight of the suspended string of pipe.
However, especially with wells that are subject to an above-average amplitude of temperature cycling, e.g. between when they are producing hot fluids and when they are shut-in or being subjected to remedial procedures, there is some prospect in conventional systems, that the pipe, in expanding, will effectively enlarge the I.D. of the annular wedge provided by the slips, but that when the pipe contracts, the pipe slips will not re-tighten, and thus substantially and perhaps will crucially diminish their gripping force on the suspended pipe string.